Male rats undernourished from the 18th day of gestation until 100 days of age were nutritionally rehabilitated until 200 days of age. Six control and six experimental rats at each of 100 and 200 days of age were killed by perfusion with buffered 2.5% glutaraldehyde. Pieces of visual cortex from each rat were postfixed in osmium tetroxide and embedded in resin. Stereological procedures at the light and electron microscopy levels were used to estimate the synapse-to-neuron ratios in cortical layers II to IV. Rats undernourished until 100 days of age had a mean +/- S.E. of 10,350 +/- 470 synapses associated with each neuron. This represented a 13% deficit (P less than 0.05) when compared to the control value of 11,950 +/- 530. Following nutritional rehabilitation till 200 days of age it was found that the previously undernourished rats had about 23% more (P less than 0.05) synapses-per-neuron than their age-matched controls. This was due almost entirely to a substantial increase in the ratio in the previously undernourished animals; the value of controls did not alter significantly between the two age groups. It appears that the deficit in the synapse-to-neuron ratio seen after a lengthy period of undernutrition is not permanent, at least in rats subsequently allowed nutritional rehabilitation. In fact, such animals seem to be capable of not only "catching-up" but "overshooting" the values found in age-matched controls.